O.A. Kayyali

Free and water soluble chloride in concrete

Abstract Two methods were used to determine the ‘free’ and ‘water soluble’ chloride ions concentration in four differing strength grade concretes. Each grade of concrete had varying concentration of Cl− admixed as NaCl. In all 24 different batches of concretes were made with admixed Cl− concentration varying from 0.2 to 2%, by weight of cement. The first method was the analysis of the pore solution expressed from specimens and the second by the analysis of the decanted solution of pulverised specimens. The results suggest that the concentration of Cl− and OH− ions as measured in the expressed pore solution represent their true free ion concentrations. Furthermore, the Cl− concentration of both the expressed and decanted solutions were found to depend on the (i) concentration of the admixed) Cl− ions; (ii) strength of the concrete; (iii) presence or absence of fly ash and (iv) presence or absence of a superplasticiser. Accordingly, there is no simple relationship between the Cl− concentration as found in decanted and pore solutions. Whereas the presence of fly ash improves the chloride binding capacity of a concrete, the presence of superplasticisers tends to lower it. The paper concludes that in order to limit the damage caused by Cl− in the concrete, an upper limit on the total Cl− contents of the admixed chloride rather than the percentage of Cl− by the weight of cement should be prescribed.

Effects of a flyash on pore solution composition in calcium and sodium chloride-bearing mortars

The influence of an Australian flyash on Cl− and OH− ion concentration of cement mortar pore solutions was investigated by preparing NaCl and CaCl2-bearing mortars with 30% replacement of cement with flyash, and analyzing the pore solutions extracted from the mortars cured for prescribed times up to 12 weeks. The flyash had little effect on Cl− ion concentration in pore solutions of mortars below at least the dosage of 2% Cl− as CaCl2. However, the addition of the flyash considerably reduced Cl− ion concentration in mortars treated with NaCl at 1 and 2% Cl− level. It was also found that the flyash raised the Cl−/OH− ratio in pore solution at the dosages up to 2% Cl− in both calcium and sodium chloride-bearing mortars.

Bond and slip of coated reinforcement in concrete

Abstract Concrete beams reinforced with black, epoxy coated or galvanized steel were tested to failure in flexure and the slip of the reinforcement was monitored. While there was clear evidence of the influence of bar deformations on the bond capacity of beams reinforced with smooth compared to ribbed black steel bars, the majority of the work concentrated on comparing the load-slip behaviour of ribbed bars as affected by the presence of surface coatings. The ultimate capacity in flexure of beams reinforced with ribbed galvanized or epoxy coated bars was not statistically different to that of black steel reinforced beams. The results from load-slip measurements were indicative of the variation in bond for the different bar coatings. It was found that loads at a slip of 0.05 mm were generally too close to the ultimate load and accordingly lower slip values in the serviceability range, i.e. 0.01 and 0.02 mm, were adopted for the analysis. From this it was found that the mean critical load at these slip values for the ribbed galvanized bars was not statistically different to the black steel. On the other hand, the load at slip for the epoxy coated ribbed bars was significantly lower, by about 20%, than that for both the black and galvanized steel bars. Overall, the results of this work indicated that there was no significant loss in bond with the use of galvanized bars, though a significant reduction was observed with epoxy coated bars.

Effect of carbonation on the chloride concentration in pore solution of mortars with and without flyash

Abstract 1% of Cl− ion by weight of cementitious materials was added as CaCl2 to mortar mixes with and without flyash. Accelerated carbonation was applied on the mortars after short and long fog curing periods. The results showed significant increase of Cl− ions in the pore solution occurring as a result of carbonation. The increase was much more pronounced in flyash mortars. Prolonged initial curing helped to retain Cl− bound within the hydration compounds in the case of plain mortars while it resulted in the increase of Cl− content in the pore solution of flyash mortars.

CHLORIDE PENETRATION AND THE RATIO OF CL-/OH- IN THE PORES OF CEMENT PASTE

Abstract Hardened Portland cement paste samples were immersed in sodium chloride solution. Their pore solution was expressed and analysed for Cl − and OH − ions concentrations. The development of free Cl − and of Cl − /OH − ratio is discussed in relation to the limits previously established for the onset of depassivation of steel reinforcement. It was found that within a relatively short period penetration of the chloride ions could cause the Cl − /OH − ratio in the pores to exceed the depassivation limits.

Mercury intrusion porosimetry of concrete aggregates

Mercury intrusion porosimetry was performed for aggregates used for concrete in Kuwait. The curves which were established helped to identify the types of rocks used. Microstructural flows in the form of pores whose diameter is larger than 100 nm were associated with certain identifiable rocks. Such pores are considered as a source of weakness for concrete and as a factor which adversely affects its durability.RésuméOn a effectué des measures au porosimètre à mercure sur des specimens de roches et de sable utilisés au Kowait pour la fabrication du béton. Les courbes obtenues ont permis d’identifier des caractéristiques de distribution dimensionnelle des roches. Il s’est révélé que le sable et le mortier agglomérés contenaient plus de 26% de pores en volume, et, sur ce pourcentage, plus de 80% présentaient un diamètre supérieur à 100 nm. La rhyolite et le quartz, malgré une porosité totale nettement inférieure, contiennent une quantité importante de pores dont le diamètre est supérieur à 100 nm. L’auteur pense que l’inclusion dans le béton de ces agglomérats de sable et de gravier, par rapport à la pâte de ciment durcie, déterminent des champs de contraintes intensifiés. Les pores dont le diamètre est supérieur à 100 nm, qui se trouvent dans ce type de roches et dans la rhyolite et le quartz, favorisent la diffusion d’ions chlorure, ce qui affaiblit la durabilité du béton.

Porosity and compressive strength of cement paste in sulphate solution

Abstract Plain portland cement paste of low w/c ratio was immersed in high to very high concentrations of sodium or magnesium sulphates. Compressive strength development was monitored up to a period exceeding two years. The pore size distribution characteristics which developed up to one year of immersion were monitored. Little effect on the compressive strength was observed after long time of immersion. The pore size distribution was generally characterised by a decrease in the volume of pores whose diameters are larger than 150 nm and an increase in the volume of pores of smaller diameters. An interpretation of the developments in porosity and strength of the hardened paste is presented.

Effect of certain mixing and placing practices in hot weather on the strength of concrete

Abstract Certain aspects of workmanship in mixing and placing of fresh concrete in hot weather affect the strength of hardened concrete. It has been found that prolonging mixing and a reasonable delay in casting may infact increase the strength of concrete, provided curing is done properly. It was also found that time of vibration should be kept to about 10s for maximum strength to be achieved. If such effects are taken into consideration when designing the mix, there would probably be no need for special measures to be made in order to reduce the temperature of the fresh mix.

The effect of practical curing methods used in Saudi Arabia on compressive strength of plain concrete — A discussion

This article discusses the paper by taryal, chowdhury and matala (see IRRD 293269). The following points are made: (a) the effectiveness of proprietary curing compounds have yet to be evaluated in the severe climate conditions described in the paper; (b) the practibility of the curing regime of covering the concrete surface by a plastic sheet followed by water spray is challenged; (c) the effect of moisture on the indicated strength of concrete is well established, and when strength developed under differing curing regimes is being compared, it is good practice to bring the specimens to the same degree of surface wetness. It was not stated whether this was done in this programme; (d) the very marginal advantage in strength gain of plain concrete specimens observed by the authors by curing with saline water should be mentioned with great reservation and caution, as curing with salt water without also washing the coarse and fine aggregate used to make it would augment the salt ion concentration in the concrete. Durability of reinforced concrete might also be decreased by this method. (TRRL)